Sugar transport. The crr mutation: its effect on repression of enzyme synthesis

The accompanying report describes phosphotransferase system-mediated repression in mutants of Salmonella typhimurium and Escherichia coli defective in Enzyme I and histidine-containing phosphate carrier protein (HPr), the general proteins of the phosphotransferase system (PTS). Such repression prevented the cells from synthesizing the catabolic systems required for utilization of the non-PTS compounds glycerol, maltose, melibiose, mannose 6-phosphate, and alpha-glycerol phosphate. This defect can be overcome by introducing a single mutation, designated crr, into the pts mutants. The pts crr double mutants can be induced to synthesize the non-PTS catabolic systems and can therefore grow on the non-PTS sugars. The crr gene is closely linked to but not part of the pts operon, and may be a regulatory gene for the operon. Assay of the PTS proteins in crr mutants showed that the only component detectably affected was a sugar-specific protein of the PTS, Factor IIIG1c, involved in the phsophorylation of glucose (and methyl alpha-glucoside). In some crr mutants Factor IIIG1c was not detected, whereas in others it was present at reduced levels. Thus the crr gene appears to code for or regulate the synthesis of this protein. In addition to the general crr mutants, several classes of sugar-specific crr mutants were isolated. For example, maltose-, melibiose-, and glycerol-specific crr mutants were isolated, each being inducible for the corresponding catabolic enzyme system but not for the others. Unlike the general crr gene, the sugar-specific crr genes do not map near the pts operon.